1. Field of the Invention
The present invention relates to a method for mechanically and thermally mounting a semiconductor chip on a support such as a heat sink.
2. Description of the Prior Art
Semiconductor die or "chips" are normally formed in multiples in a silicon wafer which is about 5 inches in diameter. The wafer is then cut into individual chips, usually no larger than about 50 square millimeters, each of which contains a large number of electronic circuit elements. Logic chips which perform arithmetic computation functions, for example the function of addition, are now in production which have more than 40,000 transistors and other circuit elements in a 50 square millimeter area.
Even with the large number of functions which can be accommodated on a single chip, a large number of such chips are required in computing systems with significant capacity. Overall system speed is limited by the necessity of transmitting signals between logic circuits on different chips, and it would be desirable to increase the size of the individual chips to reduce the number of chip-to-chip communications, and thus increase system speed. Large size chips have not been effectively implemented for a variety of reasons, among which is the difficulty in adequately mounting such a chip to a supporting structure. Such difficulties are multiplied when the size of the chips is increased to "wafer-scale" having dimensions on the order of 60.times.60 millimeters.
Semiconductor chips are typically mounted to a supporting structure by solder. Soldering of the chip to the support in the conventional fashion results in voids in the solder which contain trapped ambient gas from the soldering environment, usually air. The chip is typically moved back and forth while the solder is molten to reduce such voids, and mechanical brushing techniques have also been tried to distribute the solder and eliminate the trapped gas. However, even when great care is taken to make the soldering attachment of a relatively small, 50 square millimeter chip, about 20% or more of the volume between the chip and the support typically constitutes gas-filled voids in the solder.
Conventional solder attachment techniques are insufficient to properly mount a wafer scale chip. A wafer scale chip requires positive cooling, as by mounting the chip directly to a heat sink, because of the heat generated by the large number of circuits. The gas-filled voids in a conventional soldered connection to a heat sink result in "hot spots" because the gas in the voids is a thermal insulator. Such a hot spot in a critical position could destroy the chip. Furthermore, it is not possible as a practical matter to apply mechanical brushing techniques or movement techniques to chips having wafer scale dimensions because such chips must be extremely accurately positioned on the support and are more prone to scrubbing damage.